The present invention relates to an hydraulic system in a motor vehicle for operating a friction clutch, including a slave cylinder acting upon a spring plate of the clutch, and a manually or automatically operated master cylinder acting upon the slave cylinder.
Such systems are known and are used in motor vehicles for operating the clutch. The master cylinder may be operated directly or power-assisted by the driver via a pedal, or via an actuator such as an electric motor, for example. A control unit can operate the electric motor upon determining the operating conditions for the clutch. The control can be based on the analysis of appropriate input variables, such as, for example, the engine speed, the position of the fuel metering device, the vehicle speed, the clutch state such as the clutch's thermal condition, and the like, the transmission state and/or an input of the driver's intent. The clutch to be operated is designed in such a way that a pressure plate, which may comprise a flywheel mounted on the crankshaft of the engine, or of the secondary part of a dual-mass flywheel, or of a component attached to one of the former, in cooperation with a friction plate, which is connected to the pressure plate such that it is axially displaceable and rotatable with the pressure plate, secures a clutch disk against the action of a spring plate. The clutch disk is rotatably mounted on the input shaft of the transmission. The slave cylinder acts upon the spring plate, thus engaging and disengaging the clutch. The clutch travel path, initiated by the slave cylinder, from an engaged clutch position, via a slipping clutch, to a disengaged clutch position, exhibits a typical force characteristic of a spring plate, having an initially increasing disengagement force which passes through a maximum value, and subsequently decreases. In order to achieve a more even force characteristic over the clutch disengagement path, different designs of compensation springs on the clutch have been proposed, in particular for self-adjusting clutches. The compensation springs require axial installation space and are partly dependent on the clutch's state of wear with regard to its force deployment, so that compensation for the declining spring plate characteristic is difficult over the clutch's service life. This difficulty is similarly true for push-type and pull-type clutches.
Furthermore, it is known to provide restoring springs for the driver operated pedal. The restoring springs restore the pedal to an initial position, in a non-operated state, while the clutch is engaged.